Some hazard having been found from a sanitary standpoint in the use of propellants comprising halogenated hydrocarbons, such as Freons, there is at present a tendency of avoiding the use thereof for dispensing as an aerosol in the case of several products, such as, for example, cosmetic products. The present tendency is to follow two ways for at least partially eliminating the use of such propellants. A first way consists of using harmless propellants, such as CO.sub.2, while the second way contemplates at least partially eliminating the propellant (either CO.sub.2 or halogenated hydrocarbons) by using manual pumps allowing the liquid to be dispensed to a relatively high pressure, thus obtaining an atomization thereof when the liquid reaches the atmosphere.
The known prior art dispensing pumps made by moulding of plastics material substantially comprise two pistons moving in unison against a spring for at least some length of the stroke thereof and relatively to each other during a successive length of such a stroke. This relative movement allows the opening of a delivery valve (generally comprising a projection of one piston cooperating with a port or passage in the other piston) and accordingly the dispensing of pressurized liquid through a nozzle.
These known pumps of the prior art suffer from some disadvantages reflecting on the functionality thereof. Thus, the two pistons should be provided with seals creating substantial friction along the walls of cylinders in which such pistons are slidably mounted. In addition, the provision of seals involves the implementation of complicated and precise moulds.